In recent years, mobile electronic devices such as notebook computers and mobile phones have rapidly spread along with the development of communication systems and their performances also have improved year by year. Particularly, mobile devices are likely to have a larger power consumption along with the improvement in their performances. Then, batteries as their power source are increasingly required to have a high energy density, a large output and the like.
As batteries having a high energy density, lithium ion batteries have been developed, and widely used since the 1990s. For example, in the lithium ion batteries, a lithium-containing transition metal oxide such as lithium manganate or lithium cobaltate is used as an electrode active material for the cathode and carbon is used as an electrode active material for the anode, and charging/discharging is performed utilizing the intercalation/deintercalation reactions of lithium ions into/from the electrode active materials. Such lithium ion batteries have a high energy density and excel in cycle characteristics, and are utilized for various types of electronic devices including mobile phones. However, since the reaction rate of the electrode reaction is low, the battery performances remarkably decrease if a large current is extracted. Hence, the lithium ion batteries have a difficulty in producing a large output, and have a drawback of requiring a long time for charging.
As power storage devices capable of producing a large output, electric double-layer capacitors are known. The electric double-layer capacitors can produce a large output since they can output a large current at one time. They have excellent cycle characteristics and have been developed as a backup power source. However, they have a very low energy density, and have a difficulty in being downsized, so they are not suitable for a power source for mobile electric devices.
In order to provide an electrode material which is light and has a large energy density, also batteries using a sulfur compound or an organic compound as an electrode active material have been developed. For example, Patent Document 1 (U.S. Pat. No. 4,833,048) and Patent Document 2 (Japanese Patent No. 2715778) disclose a battery using an organic compound having disulfide bonds for the cathode. This utilizes as the principle of the battery an electrochemical redox reaction involving generation and dissociation of the disulfide bonds. Since this battery is constituted of electrode materials containing as main ingredients elements having a small specific gravity, such as sulfur and carbon, it serves for a certain effect in view of a high-energy density and large-capacity battery. However, since rebonding of the dissociated bonds exhibits a small efficiency and the electrode active material diffuses in an electrolyte, the battery has a drawback of easily decreasing in the capacity when the charge/discharge cycle is repeated.
Further as a battery utilizing an organic compound a battery using a conductive polymer as an electrode material is proposed. The battery is one utilizing as the principle the doping/dedoping reactions of electrolyte ions to/from the conductive polymer. The doping reaction refers to a reaction to stabilize charged radicals generated by oxidation or reduction of a conductive polymer, with counter ions. Patent Document 3 (U.S. Pat. No. 4,442,187) discloses a battery using such a conductive polymer as a cathode or anode material. This battery is constituted only of elements having a small specific gravity, such as carbon and nitrogen, and was expected as a high-capacity battery. However, the conductive polymer has a characteristic that charged radicals generated by redox delocalize over a broad rage of the π electron conjugate system, and interact to cause electrostatic repulsion and radical extinction. This putts a limitation on charged radicals generated, i.e., the doping concentration, and putts a limitation on the capacity of a battery. For example, it is reported that a battery using polyaniline as a cathode has a doping ratio of not more than 50% and a battery using polyacetylene has that of 7%. Although a battery using a conductive polymer as an electrode material serves for a certain effect in view of weight reduction, the battery having a large energy density has not been provided.
As a battery using an organic compound as an electrode active material of the battery, one using a redox reaction of a radical compound is proposed. For example, Patent Document 4 (Japanese Patent Application Laid-Open No. 2002-151084) discloses organic radical compounds, such as nitroxide radical compounds, aryloxy radical compounds and polymeric compounds having a specified aminotriazine structure, as an active material, and a battery using the organic radical compound as a material for a cathode or an anode. Further, Patent Document 5 (Japanese Patent Application Laid-Open No. 2002-304996) discloses a power storage device using a nitroxide compound, particularly a compound having a cyclic nitroxide structure, as an electrode active material. The polyradical compound used there as an electrode active material is synthesized by reacting and polymerizing 2,2,6,6-tetramethylpiperidine methacrylate with azobisisobutyronitrile as a polymerization initiator, and thereafter oxidizing the polymer using m-chloroperbenzoic acid. On the other hand, Patent Document 6 (Japanese Patent Application Laid-Open No. 2002-313344) discloses also a battery using a nitroxyl radical polymer being a polyradical compound as a binder for an electrode.
Meanwhile, electrodes of batteries generally contain an electric conductivity-imparting agent to enhance electron conductivity other than the active material. Proposals are made to provide batteries having a high energy density and a high output by compositing an active material and an electric conductivity-imparting agent to effectively make up for the low electron conductivity of the active material. For example, Patent Document 7 (Japanese Patent Application Laid-Open No. 2003-292309) discloses a composite in which the surface of particles composed of lithium iron phosphate is coated with a conductive carbon.
On the other hand, as a method for fabricating an electrode for a polymer secondary battery, a heat press method is disclosed. Patent Document 8 (Japanese Patent Application Laid-Open No. 001-118570) discloses an electrode producing method in which a mixed powder of a polymer active material powder and an electric conduction-aiding agent powder is heat pressed.
Patent Document 9 (Japanese Patent Application Laid-Open No. 2002-298850) discloses a battery in which an active material containing a radical compound is particles composed of two or more compositions. It is contended that coating the surface of a conductive material with a radical compound enlarge the surface area of the radical compound and can provide a high output density. However, the case of coating a conductive material with a radical compound cannot provide a sufficient output density because electron conduction paths of the conductive material are not formed externally. Patent Document 9 further contends that binding a radical compound and a conductive material with a polymeric material holds the binding even if particles are deformed in battery operation and provides an excellent cycle life. However, there arises a problem that the polymeric material used as a binder lacks electron conductivity, causing an increase in the electrode resistance and causing a decrease in the output density.
Patent Document 1: U.S. Pat. No. 4,833,048
Patent Document 2: Japanese Patent No. 2715778
Patent Document 3: U.S. Pat. No. 4,442,187
Patent Document 4: Japanese Patent Application Laid-Open No. 2002-151084
Patent Document 5: Japanese Patent Application Laid-Open No. 2002
Patent Document 6: Japanese Patent Application Laid-Open No. 2002-313344
Patent Document 7: Japanese Patent Application Laid-Open No. 2003-292309
Patent Document 8: Japanese Patent Application Laid-Open No. 2001-118570
Patent Document 9: Japanese Patent Application Laid-Open No. 2002-298850